Sidearm having first round in place (6)
I believe the answer is:
pistol
'sidearm' is the definition.
(type of firearm)
'first round in place' is the wordplay.
I cannot quite see how this works, but
'first' could be 'ist' (ist resembles 1st) and 'ist' is present in the answer.
'round' could be 'o' (o is a circular letter) and 'o' is found within the answer.
'place' could be 'pl' (abbreviation used in postal addresses) and 'pl' is found in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'having' acts as a link.
Can you help me to learn more?
(Other definitions for pistol that I've seen before include "Shakespearean - weapon" , "Shakespearean character" , "making an appearance in Shakespeare's King Henry V" , "Hand-held firearm" , "Rectangular" .)